1. Field of the Invention
The present invention relates generally to a pulse width modulated signal amplifier, and more directly to a pulse width modulated signal amplifier of the type which drives a load by a pair of drive signals in complementary phase relationship to each other.
2. Description of the Prior Art
In a prior art Class B amplifier in which a load is driven in the BTL (balanced transformerless) system, a pair of pulse modulators are used to modulate a signal with a pair of modulating signals, which are opposite in phase with each other, to produce a pulse width modulated positive signal and a pulse width modulated negative signal reverse in phase from the former. These signals are then supplied through low pass filters to both ends of the load.
In order to obtain a pulse width modulated signal, in general, as shown in FIG. 1A, a sawtooth waveform signal S.sub.o is sliced with a modulating signal, for example, a low frequency signal S.sub.a, to produce a pulse width modulated signal P.sub.a shown in FIG. 1B. In the prior art, the same sawtooth waveform signal S.sub.o is sliced at the same time with a low frequency signal S.sub.b which is opposite in phase from the former to produce a pulse width modulated signal P.sub.b shown in FIG. 1C. By this method, however, both the pulse modulated signals P.sub.a and P.sub.b cannot be generated perfectly opposite in phase. As a result, noises cannot be substantially removed. Furthermore, in the prior art, two systems of negative feedback loops are necessary for the respective modulators, which results in complicated circuit construction.